DESCRIPTION (applicant's abstract): The goal of this proposal is to examine the role of social cues in the seasonal growth of the brain nuclei that control singing behavior in adult songbirds. Previous experiments have established that male white-crowned sparrows exposed to reproductively active females will develop higher plasma testosterone levels and larger song control nuclei than males without females. The proposed experiments will explore both the nature of the social cues and the mechanism by which such cues affect song system morphology and singing behavior. Three hypotheses will be tested: (i) auditory stimulation is necessary for socially-induced changes in song system morphology and singing behavior. (ii) the effect of social signals on song system and morphology is separable from the effect of gonadal steroids on these measure. and (iii) the effect of social signals on the morphology of the song nucleus HVc is mediated in part by an increase in neurogenesis. I will address these hypotheses by making comparisons of song system behavior in socially stimulated and unstimulated males treated under several different regimens: (i) hearing males vs. deafened males, (ii) T-implanted males vs. blank implanted males, and (iii) males given [3H]-thymidine to mark newly generated neurons. The studies will increase our knowledge of the ways in which animals coordinate reproductive effort, and will refine our understanding of the role of neural integration of signals from the internal and external environments in the control of reproduction. Also, this research will provide insights into the mechanisms underlying plasticity of the adult brain.